The present invention generally relates to a system for defeating erroneous correction in a digital signal reproducing apparatus, and more particularly to a system for defeating erroneous correction in a digital signal reproducing apparatus using an adjacent error correction which uses error pointers. Such error pointers may be obtained by an error detection code to perform corrections of up to two words (vectors) in one block, where the system for defeating erroneous correction is capable of preventing erroroneous correction from being performed and further reducing the abnormal noise generated upon erroneous correction, by stopping the above adjacent code correction operation when the number of words (vectors) which are error-detected by the above error detection code and partial syndromes respectively assume certain values or until the member of error-detecting words and the partial syndromes respectively assume certain values in a following block.
In known recording and reproducing apparatus which record and reproduce an analog audio signal on and from a travelling magnetic tape by means of a stationary head, there have unavoidably been a number of problems such as the introduction of wow and flutter in the tape travel and noise and distortion arising in the tape and head systems. As a natural consequence, these problems impose limitations when improving the quality of recording and reproducing audio signals.
Accordingly, a method which converts an analog audio signal into a modulated digital signal (hereinafter simply referred to as "digital signal") by a modulation method such as pulse code modulation (PCM) and recording and reproducing this digital signal has been developed. By this method, problems such as those relating to the S/N ratio and distortion due to non-linearity of the recording medium are solved to a considerable degree.
In order to record and reproduce this digital signal, recording and reproducing systems having a wider frequency band or greater number of channels in comparison with those of a system for recording and reproducing analog signals, are required. Accordingly, a method in which a so-called video tape recorder (hereinafter referred to as VTR) which has heretofore been widely sold on the market as an apparatus for recording and/or reproducing composite video signals is used as the above recording and reproducing apparatus, has been realized. The term "composite video signal" is used in the present specification to designate a signal which results from the addition of synchronizing signals such as a vertical synchronizing signal, equalizing pulses, and a horizontal synchronizing signal to a video information signal. A VTR of this type is capable of recording and/or reproducing video signals having a wide frequency band by means of rotary heads which trace with a relatively high relative tracing speed over oblique tracks on a traveling magnetic tape.
Recording of an audio signal by using this VTR is accomplished by coverting the analog audio signal which is to be recorded into a digital signal, and inserting this digital signal between synchronizing signals which are the same as the synchronizing signals of an ordinary composite video signal, in an adapter apparatus connected to the VTR, to supply the composite digital signal thus obtained to the VTR, and record this signal on the magnetic tape by means of rotary heads. At the time of reproduction, the signal reproduced from the magnetic tape by the rotary heads in the VTR is supplied to the adapter apparatus, wherein the synchronizing signals are eliminated, and the resulting digital signal is converted into an analog signal, thereby being restored to the original audio signal.
In the case where dust particles are adhered to the surface of the above mentioned magnetic tape, or in the case where irregularities exist in the magnetic material of the magnetic tape, a signal loss or dropout occurs in one portion of the reproduced signal. In the case where such signal dropout occurs, and the most significant bit (MSB) is not reproduced, for example, the signal obtained by decoding will have a considerably large error value. If this signal is converted into an analog signal and reproduced, as it is, it will be accompanied by a high noise voltage, and the reproduced sound will have an unsatisfactory sound quality.
As a countermeasure, an interleaving system has heretofore been adopted. In a digital signal processing system adopting this interleaving system, the input analog signal is sampled every appropriate period in time in a sampling-and-holding circuit, and the resulting sampled signal is converted into a modulated digital signal in an analog to digital (A/D) converter. This modulated digital signal is fed into a memory in which the data write-in and read-out operations are controlled by a control pulse, and a sequence of word groups are arranged where the words obtained for every one sample of the digital signal respectively interleave with each other being separated from each other by a period of 10-odd H (H denotes one horizontal synchronizing period). Here, "one word" refers to the combination of bits obtained from one sample. The period of time represented by 10-odd H, is determined in conformance with the format.
A composite synchronizing signal is added to the signal thus obtained to produce a composite digital signal, which is then recorded on a magnetic tape by a VTR. This rearrangement of the order of data is referred to as "interleaving". Since the interleaving of data results in the distribution of the signal information, even when the recording or reproducing signal is deficient over one or more horizontal scanning (H) periods due to dropout, a related signal existing in another horizontal scanning period may be used to restore the original information.
A data part is extracted from the signal reproduced from the VTR. This data part, which undergoes wave-shaping, is then converted into a binary coded digital signal and thereafter supplied to a memory, where the signal are written in and read out responsive to a control pulse and restored back into the original order. This restoring of the signal back into the original order is hereinafter called "de-interleaving". The resulting signal is supplied to a digital to analog (D/A) converter where it is restored back into the original analog signal.
Thus, when a recording and/or reproducing system employing the above described VTR is used, a large number of code errors in the digital signal are generated, when dropout, jitter, noise, fluctuation in the reproduction level, interference between the codes, and the like exist due to dust particles and scratches existing on the magnetic tape. There are two types of code errors in the digital signal, mainly, the random error which is generated in a random manner, and the burst error which is generated in a bursted manner. Accordingly various types of code correction systems have been proposed which perform code correction upon de-interleaving with respect to these errors.
However, there has not been an effective sytem which prevents the erroneous correction in these error correction systems.